1. Field of the Invention
The present invention relates to lasers and particularly to an efficient and compact tunable optical source of high peak power coherent radiation at a desired wavelength through the use of a tuning device, modelocking, a high power large active area semiconductor amplifier and non-linear frequency conversion.
2. Description of the Related Art
Of all laser types, semiconductor lasers are the most attractive for commercial applications due to their high efficiency, compactness, low manufacturing costs, long operating lifetime, and wide choice of available wavelengths. However, many applications, such as nonlinear frequency conversion and free space and fiber optical communications, require peak powers which are well beyond the output range of not only conventional narrow stripe laser diodes, but also the recently developed diffraction limited, large active area semiconductor amplifiers. Large increases in the peak laser diode power can be achieved through the use of modelocking. Modelocked pulses, directly generated by narrow stripe semiconductor amplifiers are, however, limited to average powers below 10 milliwatts (mW), pulse energies below 10 picojoules (pJ), and peak powers below 1 watt (W). These power levels fall short by one to two orders of magnitude from those required for efficient frequency conversion.
Another approach for increasing the output power has been to use high power laser diode arrays as the active element. The average output powers however, have been relatively small, typically &lt;10 mW. This is a consequence of the fact that large area active elements require high pump currents, and the effective modulation depth of the gain element is relatively small due to the limited available RF power of .apprxeq.1 W. As a result, active modelocking can be achieved only near laser threshold, where output powers are low.
The only available lasers emitting in the commercially important blue spectral region are gas lasers such as the Argon ion laser, and the HeCd laser. Gas lasers suffer from several major deficiencies limiting their applications, particularly a very low electrical-to-optical power conversion efficiency of &lt;0.01%. A laser with low efficiency requires high electrical power to generate optical powers above a few milliwatts, and removal of large amount of excess heat. For powers above .apprxeq.100 mW, liquid cooling has to be used, which is unattractive for many applications. Although low power blue gas lasers have found several applications such as high speed laser printing, IC wafer alignment, IC mask generators, CD mastering systems, they suffer from relatively short lifetime of roughly 5,000 hours, mechanical vibration caused by the air fan, and relatively large cost in the range of $5,000-$25,000. These limitations prevent gas lasers from gaining wider commercial utilization.
Numerous solid state alternatives to the gas lasers have been proposed and demonstrated, including fiber upconversion lasers, directly frequency doubled laser diodes, sum frequency mixing between a Nd:YAG laser and a laser diode. These systems, however, have generated only limited blue output powers below approximately 50 mW, and require complex and expensive resonant cavities to achieve acceptable conversion efficiencies. The relatively low powers are due to the low power available from conventional laser diodes of &lt;100 mW, and the resulting low nonlinear conversion efficiency.